The present invention relates to the development and application of chemo-thermo-piezoresistive smart cement with bulk sensing properties to measure the changes in the electrical properties of the smart cement in order to monitor its integrity and performance in real-time.
Oil well cement serves many purposes in the deep water drilling projects. Foremost important among these is to form a sealing layer between the well casing and the geological formation referred to as the zonal isolation. For successful oil well and gas well cementing operations, it is critical to determine the flowing of cement slurry between the casing and formation, depth of the circulation losses and fluid loss, setting of cement in place and performance of the cement after hardening. In the civil infrastructures (foundations, piles, pipelines, bridges, highways, storage facilities and buildings) Portland cement serves many purposes for successful construction and/or repairing applications. Hence it is critical to determine the hardening of the cement and monitoring the conditions in the cement throughout the entire service life.
Two studies done on blowouts on the U.S. outer continental shelf (OCS) during the period of 1971 to 1991 and 1992 to 2006 clearly identified cementing failures as the major cause for blowouts (Izon et al. 2007). Cementing failures increased significantly during the second period of study when 18 of the 39 blowouts were due to cementing problems (Izon et al. 2007). Also the deep-water horizon blowout in 2010 in the Gulf of Mexico was due to cementing issues (Kyle et al. 2014). With some of the reported failures and growing interest in environmental and economic concerns in the oil and gas industry and civil infrastructures, integrity of the cemented materials are of major importance. Therefore, proper monitoring and tracking the entire process of well cementing and other cementing operations become important to ensure cement integrity during the service life (Vipulanandan et al. 2014a-d). At present there is no technology available to monitor cementing/coating/concreting operations in real time from the time of placement through the service life of the applications. Also during the oil and gas well installation, there is no reliable method to determine the length of the competent of cement supporting the casing.
The API and ASTM tests for cementing include procedures for finding density, free water, fluid loss, rheological properties and compressive strength. All these tests are important for composing a successful cement grout, but most of them consider only one (thickening time) or a few points of time during the setting process. Several non-destructive methods (X-ray diffraction, calorimetric analysis, scanning electron microscopy and ultrasonic methods) have been used by researchers to monitor the curing and characterize the behavior of cementitious materials (Vipulanandan et al. 2014a,b). Electrical resistivity measurement has been used by many researchers for characterizing concrete and cementitious grouts for various applications (McCarter 1996; Wei et al. 2008; Azhari et al. 2012; Han et al. 2012; Vipulanandan et al. 2014a-c; Liao et al. 2014). The advantages in using the electrical resistivity to characterize the material include its sensitivity to changes and relatively easy measure. Electrical resistivity of cement is affected by a number of factors such as pore structure, pore solution composition, cementitious content, w/c ratio, moisture content and temperature (McCarter 1994; Vipulanandan et al. 2014a,b). Electrical conduction occurs primarily due to ion transport through the pore solution in a cement-based system and hence strongly depends on both pore solution conductivity and porosity (Wei et al. 2008). Therefore chemical reactions and change in microstructure of cement during the hydration process affects the electrical resistivity response of the cement-based composites (Zuo et al. 2014; Vipulanandan et al. 2014b).
Past studies have reported that the interfacial factors are important in obtaining electrical resistivity from electrical resistance (Chung 2001). Due to the voltage present during electrical resistance measurement, electric polarization occurs as the resistance measurement is made continuously. The polarization results in an increase in the measured resistance. The conventional methods of measuring the electrical resistivity of cementitious materials can be categorized into direct-current (DC) methods and alternating-current (AC) methods, both of which require electrodes for their measurements. Therefore, there is the potential for contact problems between the electrodes and the matrix, which could completely affect the accuracy of the measurement. Recent studies have suggested that replacing the DC measurement with the AC measurement can eliminate the polarization effect (Zhang et al. 2010, Vipulanandan et al. 2013). It has been observed that the relationship between resistivity and curing time for various types of cement grouts followed a similar pattern (Wei et al. 2008; Vipulanandan et al. 2014a-c). The electrical resistivity dropped to a minimum value, and then gradually increased with time. Initially after mixing cement with water, resistivity decreased to a minimum value (ρmin), and the corresponding time to reach the minimum resistivity was (tmin). The tmin can be used as an index for the speed of chemical reactions and cement setting times. Also the electrical resistivity is predominated by the conductivity of the pore solution and the connectivity of pores Immediately after mixing, the pores are connected and more conduction paths are formed between cement particles. After 24 hours of curing the hydration products block the conduction path and tortuosity increases. The decrease of connectivity of pores results in a sharp increase in the resistivity curve (Wei et al. 2008; Vipulanandan et al. 2014a-c). However, there is very limited information in the literature about quantification of the electrical resistivity during curing of the oil well cements. As the porosity decreases due to shrinkage and increased accumulation of hydration products in the cement grout pores there is increase in compressive strength.